Specify target files from command line
Blender executes the command line options in the over they are given, so Blender sees the command to render (-a) and then sets the command to set the output path (-o). This means the output path gets set after the render has happened.

You just need to swap the order of the commands so you have:

blender -b knob.blend -s 1 -e 2 -o //__targets/knob_#.png -a